Low Complexity Maximum Likelihood Detection of Concatenated Space Codes for Wireless Applications

ABSTRACT

Good transmission characteristics are achieved in the presence of fading with a transmitter that employs a trellis coder followed by a block coder. Correspondingly, the receiver comprises a Viterbi decoder followed by a block decoder. Advantageously, the block coder and decoder employ time-space diversity coding which, illustratively, employs two transmitter antennas and one receiver antenna.

CROSS-REFERENCE TO RELATED APPLICATION(S)

This application is a continuation of U.S. patent Ser. No. 13/691,505,filed Nov. 30, 2012, which is a continuation of U.S. patent applicationSer. No. 12/650,007, filed Dec. 30, 2009 (now U.S. Pat. No. 8,351,545),which is a continuation of Ser. No. 11/018,780, filed Dec. 21, 2004 (nowU.S. Pat. No. 7,643,568), which is a continuation of U.S. patentapplication Ser. No. 10/334,343, filed Dec. 30, 2002 (now U.S. Pat. No.6,853,688), which is a continuation of U.S. patent application Ser. No.10/005,095, filed Dec. 3, 2001 (now U.S. Pat. No. 6,807,240), which is adivisional of U.S. patent application Ser. No. 09/167,422, filed Oct. 5,1998 (now U.S. Pat. No. 6,501,803), which claims the benefit of U.S.Provisional Application No. 60/063,794, filed Oct. 31, 1997, all ofwhich are incorporated herein by reference in their entirety.

BACKGROUND OF THE INVENTION

This invention relates to wireless communication and, more particularly,to techniques for effective wireless communication in the presence offading and other degradations.

The most effective technique for mitigating multipath fading in awireless radio channel is to cancel the effect of fading at thetransmitter by controlling the transmitter's power. That is, if thechannel conditions are known at the transmitter (on one side of thelink), then the transmitter can pre-distort the signal to overcome theeffect of the channel at the receiver (on the other side). However,there are two fundamental problems with this approach. The first problemis the transmitter's dynamic range. For the transmitter to overcome an xdB fade, it must increase its power by x dB which, in most cases, is notpractical because of radiation power limitations, and the size and costof amplifiers. The second problem is that the transmitter does not haveany knowledge of the channel as seen by the receiver (except for timedivision duplex systems, where the transmitter receives power from aknown other transmitter over the same channel). Therefore, if one wantsto control a transmitter based on channel characteristics, channelinformation has to be sent from the receiver to the transmitter, whichresults in throughput degradation and added complexity to both thetransmitter and the receiver.

Other effective techniques are time and frequency diversity. Using timeinterleaving together with coding can provide diversity improvement. Thesame holds for frequency hopping and spread spectrum. However, timeinterleaving results in unnecessarily large delays when the channel isslowly varying. Equivalently, frequency diversity techniques areineffective when the coherence bandwidth of the channel is large (smalldelay spread).

It is well known that in most scattering environments antenna diversityis the most practical and effective technique for reducing the effect ofmultipath fading. The classical approach to antenna diversity is to usemultiple antennas at the receiver and perform combining (or selection)to improve the quality of the received signal.

The major problem with using the receiver diversity approach in currentwireless communication systems, such as IS-136 and GSM, is the cost,size and power consumption constraints of the receivers. For obviousreasons, small size, weight and cost are paramount. The addition ofmultiple antennas and RF chains (or selection and switching circuits) inreceivers is presently not feasible. As a result, diversity techniqueshave often been applied only to improve the up-link (receiver to base)transmission quality with multiple antennas (and receivers) at the basestation. Since a base station often serves thousands of receivers, it ismore economical to add equipment to base stations rather than thereceivers.

Recently, some interesting approaches for transmitter diversity havebeen suggested. A delay diversity scheme was proposed by A. Wittneben in“Base Station Modulation Diversity for Digital SIMULCAST,” Proceedingsof the 1991 IEEE Vehicular Technology Conference (VTC 41st), pp.848-853, May 1991, and in “A New Bandwidth Efficient Transmit AntennaModulation Diversity Scheme For Linear Digital Modulation,” inProceedings of the 1993 IEEE International Conference on Communications(IICC '93), pp. 1630-1634, May 1993. The proposal is for a base stationto transmit a sequence of symbols through one antenna, and the samesequence of symbols—but delayed—through another antenna.

U.S. Pat. No. 5,479,448, issued to Nambirajan Seshadri on Dec. 26, 1995,discloses a similar arrangement where a sequence of codes is transmittedthrough two antennas. The sequence of codes is routed through a cyclingswitch that directs each code to the various antennas, in succession.Since copies of the same symbol are transmitted through multipleantennas at different times, both space and time diversity are achieved.A maximum likelihood sequence estimator (MLSE) or a minimum mean squarederror (MMSE) equalizer is then used to resolve multipath distortion andprovide diversity gain. See also N. Seshadri, J. H. Winters, “TwoSignaling Schemes for Improving the Error Performance of FDDTransmission Systems Using Transmitter Antenna Diversity,” Proceedingsof the 1993 IEEE Vehicular Technology Conference (VTC 43rd), pp.508-511, May 1993; and J. H. Winters, “The Diversity Gain of TransmitDiversity in Wireless Systems with Rayleigh Fading,” Proceedings of the1994 ICC/SUPERCOMM, New Orleans, Vol. 2, pp. 1121-1125, May 1994.

Still another interesting approach is disclosed by Tarokh, Seshadri,Calderbank and Naguib in U.S. application Ser. No. 08/847,635, filedApr. 25, 1997, now U.S. Pat. No. 6,115,427, (based on a provisionalapplication filed Nov. 7, 1996), where symbols are encoded according tothe antennas through which they are simultaneously transmitted, and aredecoded using a maximum likelihood decoder. More specifically, theprocess at the transmitter handles the information in blocks of M1 bits,where M1 is a multiple of M2, i.e., M1=k*M2. It converts each successivegroup of M2 bits into information symbols (generating thereby kinformation symbols), encodes each sequence of k information symbolsinto n channel codes (developing thereby a group of n channel codes foreach sequence of k information symbols), and applies each code of agroup of codes to a different antenna.

Yet another approach is disclosed by Alamouti and Tarokh in U.S.application Ser. No. 09/074,224, filed May 5, 1998, now U.S. Pat. No.6,185,258, and titled “Transmitter Diversity Technique for WirelessCommunications” where symbols are encoded using only negations andconjugations, and transmitted in a manner that employs channeldiversity.

Still another approach is disclosed by the last-mentioned inventors in aU.S. application filed Jul. 14, 1998, based on provisional 60/052,689filed Jul. 17, 1997, titled “Combined Array Processing and Space-TimeCoding,” where symbols are divided into groups, where each group istransmitted over a separate group of antennas and is encoded with agroup code C that is a member of a product code.

SUMMARY

An advance in the art is realized with a transmitter that employs atrellis coder followed by a block coder. Correspondingly, the receivercomprises a Viterbi decoder followed by a block decoder. Advantageously,the block coder and decoder employ time-space diversity coding which,illustratively, employs two transmitter antennas and one receiverantenna.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 presents a block diagram of an embodiment in conformance with theprinciples of this invention.

DETAIL DESCRIPTION

FIG. 1 presents a block diagram of an arrangement comporting with theprinciples of this invention. It comprises a trellis code modulation(TCM) encoder 10 followed by a two-branch space block encoder 20. Theoutput is applied to antenna circuitry 30, which feeds antenna 31, andantenna 32. FIG. 1 shows only two antennas, but this is merelyillustrative. Arrangements can be had with a larger number of antennas,and it should be understood that the principles disclosed herein applywith equal advantage to such arrangements.

TCM encoder 10 generates complex numbers that represent constellationsymbols, and block encoder 20 encodes (adjacent) pairs of symbols in themanner described in the aforementioned Ser. No. 09/074,224 application.That is, symbols s₀ and s₁, forming a pair, are sent to antenna 31 andantenna 32, respectively, and in the following time period symbols −s₁*and s₀* are sent to antennas 31 and 32, respectively. Thereafter,symbols s₂ and s₃ are sent to antenna 31 and 32, respectively, etc.Thus, encoder 20 creates channel diversity that results from signalstraversing from the transmitter to the receiver at different times andover different channels.

The signals transmitted by antennas 31 and 32 are received by a receiverafter traversing the airlink and suffering a multiplicative distortionand additive noise. Hence, the received signals at the two consecutivetime intervals during which the signals s₀, s₁, −s₁*, and s₀* are sentcorrespond to:

r ₀(t)=h ₀ s ₀ +h ₁ s ₁ +n ₀,  (1)

and

r ₁(t)=h ₁ s ₀ *−h ₀ s ₁ *+n ₁,  (2)

where h₀ represents the channel from antenna 31, h₁ represents thechannel from antenna 32, n₀ is the received noise at the first timeinterval, and n₁ is the received noise at the second time interval.

The receiver comprises a receive antenna 40, a two-branch space blockcombiner 50, and a Viterbi decoder 60. The receiver also includes achannel estimator; but since that is perfectly conventional and does notform a part of the invention, FIG. 1 does not explicitly show it. Thefollowing assumes that the receiver possesses {tilde over (h)}₀ and{tilde over (h)}₁, which are estimates of h₀ and h₁, respectively. Thus,the received signals at the first and second time intervals are combinedin element 50 to form signals

{tilde over (s)} ₀ ={tilde over (h)} ₀ *r ₀ +{tilde over (h)} ₁ r₁*  (3)

and

{tilde over (s)} ₁ ={tilde over (h)} ₁ *r ₀ −{tilde over (h)} ₀ r₁*,  (4)

and those signals are applied to Viterbi decoder 60.

The Viterbi decoder builds the following metric for the hypothesizedbranch symbol s₁ corresponding to the first transmitted symbol s₀:

M(s ₀ ,s _(i))=d ² [{tilde over (s)} ₀,(|{tilde over (h)}₀|² +|{tildeover (h)} ₁|²)s _(i)].  (5)

Similarly, the Viterbi decoder builds the following metric for thehypothesized branch symbol s_(i) corresponding to the first transmittedsymbol s₁:

M(s ₁ ,s _(i))=d ² [{tilde over (s)} ₁,(|{tilde over (h)} ₀|² +|{tildeover (h)} ₁|²)s _(i)].  (6)

(Additional metrics are similarly constructed in arrangements thatemploy a larger number of antennas and a correspondingly largerconstellation of signals transmitted at any one time.) If Trellisencoder 10 is a multiple TCM encoder, then the Viterbi decoder buildsthe following metric:

M└(s ₀ ,s ₁),(s _(i) ,s _(j))┘=M(s ₀ ,s _(i))+M(s _(1,) s _(j)),  (7)

or equivalently,

M[(s ₀ ,s ₁),(s _(i) ,s _(j))]=d ²(r ₀ ,{tilde over (h)} ₀ s _(i)+{tilde over (h)} ₁ s _(j))+d ²(r ₁ ,{tilde over (h)} ₁ s _(i) *−{tildeover (h)} ₀ s* _(j)).  (8)

The Viterbi decoder outputs estimates of the transmitted sequence ofsignals.

The above presented an illustrative embodiment. However, it should beunderstood that various modifications and alternations might be made bya skilled artisan without departing from the spirit and scope of thisinvention.

What is claimed is:
 1. An apparatus comprising: a trellis encoder thatgenerates a first symbol and a second symbol; a block encoder coupled toreceive the first and second symbols and generate a block of symbols,the block of symbols including the first symbol, the second symbol, acomplex conjugate of the first symbol and a negative complex conjugateof the second symbol; and a first antenna and a second antenna coupledto the block encoder to transmit the block of symbols, the first symboland the complex conjugate of the first symbol being transmitted withspace diversity and with time diversity and the second symbol and thenegative complex conjugate of the second symbol being transmitted withspace diversity and with time diversity.
 2. The apparatus as recited inclaim 1 wherein the first symbol and the complex conjugate of the firstsymbol are transmitted during different time intervals and overdifferent ones of the first and second antennas to achieve spacediversity and time diversity and the negative complex conjugate of thesecond symbol and the second symbol are transmitted during differenttime intervals and over different ones of the first and second antennasto achieve space diversity and time diversity.
 3. The apparatus asrecited in claim 2 wherein the first and second symbols are transmittedduring a first time interval and the complex conjugate of the firstsymbol and the negative complex conjugate of the second symbol aretransmitted during a second time interval.
 4. The apparatus as recitedin claim 1 further comprising a receiver comprising: a space blockcombiner configured to receive the transmitted block of symbols andsupply output signals.
 5. The apparatus as recited in claim 4 furthercomprising a Viterbi decoder coupled receive the output signals from thespace block combiner.
 6. A method comprising: trellis encoding receiveddata to generate a first symbol and a second symbol; block encoding thefirst and second symbols to generate a block of symbols that includesthe first and second symbols, a negative complex conjugate of the secondsymbol, and a complex conjugate of the first symbol; and transmittingthe block of symbols by transmitting the first symbol and the complexconjugate of the first symbol at different times and over differentchannels and transmitting the second symbol and the negative complexconjugate of the second symbol at different times and over differentchannels.
 7. The method as recited in claim 6 further comprising:transmitting the first symbol and the complex conjugate of the firstsymbol at different times and over different channels and transmittingthe second symbol and the negative complex conjugate of the secondsymbol at different times and over different channels by transmittingduring a first time period the first symbol over a first antenna and thesecond symbol over a second antenna, and transmitting during a secondtime period the negative complex conjugate of the second symbol over thefirst antenna and the complex conjugate of the first symbol over thesecond antenna.
 8. A receiver comprising: a space block combinerconfigured to receive a transmitted block of symbols and supply outputsignals, the transmitted block of symbols including a first symbol, asecond symbol, a complex conjugate of the first symbol and a negativecomplex conjugate of the second symbol; and a Viterbi decoder coupledreceive the output signals from the space block combiner.
 9. Thereceiver as recited in claim 8 wherein the output signals of the spaceblock combiner are where {tilde over (s)}₀={tilde over (h)}₀*r₀+{tildeover (h)}₁r₁*, and {tilde over (s)}₁={tilde over (h)}₁*r₀−{tilde over(h)}₀r₁*, where {tilde over (h)}₀ is a channel estimate of a firstchannel from a first transmitting antenna to the receiver and {tildeover (h)}₁ is a channel estimate of a second channel from a secondtransmitting antenna to the receiver, and r₀ is a first signal receivedat a first time period and transmitted over the first and secondchannels and r₁ is a second signal received at a second time period andtransmitted over the first and second channels.
 10. The receiver asrecited in claim 9 where, r₀=h₀s₀+h₁s₁+n₀ and r₁=h₁s₀*−h₀s₁*+n₁, whereh₀ is the first channel, h₁ is the second channel, s₀ is the firstsymbol, s₁ is the second symbol, n₀ is received noise during the firsttime period, and n₁ is received noise during the second time period.